1. Field of the Invention
The invention relates to a flexible graphite layer material which contains non-graphitic corrosion inhibitors.
2. Description of the Prior Art
The manufacture of flexible graphite layer material is known and according to German Pat. No. 66 804 this is accomplished by dipping flaky, natural graphite particles into a liquid oxidant and heating them subsequently to about 1000.degree. C. within a few seconds. The individual graphite particles expand in this treatment by a considerable amount and can be densified without the addition of a special binder into flexible planar structures such as thin foils or thicker laminar shapes, as for instance, by rolling, pounding or hammering. The layer material, which is impervious to liquids and gases, and also temperature-resistant and self-lubricating, is used among other things as a seal or packing, where the layer material is in contact with other materials such as metals.
It has been found that some types of steel corrode in contact with graphite layer material. The intensity of the corrosion attack is determined by the type of surrounding medium and the type and amount of the impurities contained in the layer material. For instance, chrome-nickel steels corrode in contact with graphite layer material in fully demineralized water, but not in ordinary tap water. It is known to prevent harmful corrosion of certain types of steel in contact with graphite layer material or to reduce it considerably by working alkali metal salts of one or several elements of group VI A of the periodic system of the elements into the layer material. The effectiveness of these inhibitor additives, however, is limited in time, since the salts gradually go in solution or are washed out, especially if the graphite layer materials are used in high-pressure steam. For instance, in a packing, the corrosion inhibitor content of which was originally about 0.8% by weight, a migration of the inhibitors after extended exposure to steam of 50 bar, 300.degree. C. was measured in accordance with the following Table:
______________________________________ Inhibitor Content (sodium molybdate) ______________________________________ Inside 0.09% by weight 0.07 0.09 0.22 0.50 1.00 Outside 2.70 ______________________________________
The steel in contact with the inside of the packing showed pitted corrosion damage after the test was terminated.